Paper mills produce paper through a wood pulping process that begins with wood chips as the raw material. The wood chips are initially broken down or digested into wood fibers, which are then passed on for further processing to produce a variety of paper-based products. During the process of “digesting” the wood chips, a caustic solution referred to as white liquor is added to the wood chips. The white liquor is typically composed of mainly sodium sulfide (Na2S) and sodium hydroxide (NaOH). At the end of the “digesting” process, chemical reactions, which have produced the desired wood fibers, have also produced a different caustic solution which is now referred to as black liquor. As a secondary process to minimize waste and optimize production, the black liquor may be recovered and chemically converted first to a green liquor and then back to a white liquor. The recycled white liquor may now be re-used in the digestion of another batch of wood chips.
The black liquor may be burned in a boiler, which leaves a smelt. This smelt is dissolved in water or “weak wash liquor” to produce green liquor. This conversion is often referred to as the “causticizing process” or the “causticizing reaction.” The causticizing process takes place in a “slaker” where “causticizing agents,” such as lime, are added during the conversion back to white liquor. The causticizing process can be optimized by proper control of the flow rate of the green liquor into the slaker and proper control of the quantity and frequency of the addition of the causticizing agent.
During the processing it is extremely beneficial to monitor the chemical composition of the changing liquor. One method of controlling the composition of the liquor during the causticizing process is to embed electrodes in the tank containing the liquor. This method is effective; however, the caustic solution that surrounds the electrodes can have adverse effects on the electrodes, such as fluctuations or inaccurate readings from the electrodes and physical deterioration of not only the electrode surfaces but also any seals around the electrodes. Both of these seals have short lifespans when exposed to the caustic pulp liquor. Any required maintenance and/or replacement of the electrodes may involve manual control of the liquor composition or a suspension of the paper mill processing. Accordingly, there exists a need to extend the usable life of the electrodes used to measure the characteristics of the highly caustic pulp liquor.
Some methods to isolate the electrodes from the liquor include the use of elastomeric or glass seals. Both of these seals may have short lifespans when exposed to the caustic pulp liquor. Any required maintenance and/or replacement of the electrodes may involve manual control of the liquor composition or a suspension of the paper mill processing. Accordingly, there exists a need to extend the usable life of the electrodes used to measure the characteristics of the highly caustic pulp liquor and provide for the replacement of the electrodes.